666 PROCUMBENT AND FLOATING STEMS. 



freely floating ends ascend into the better illuminated upper layers of water, 

 or leaves with large blades and elongated stalks spring from the short stems, 

 and the stalks continue to grow until the plate-like blades have reached the 

 surface, where, floating, they can enjoy the full sunlight. There are also some 

 plants not fixed but swimuiiug close to the surface. These sink down to the 

 bottom only when the activity of their leaves is suspended and here for a time 

 they pass a dormant period. 



We mention here the most noticeable variations which are made use of for 

 dividing the stem-forming water-plants into architectural groups. First of all is 

 a group of plants, of which the Grass Wracks (Zostera) may be taken as a type. 

 These have stems embedded in the mud, creeping, and anchored by root fibres. 

 The leaves arising from these stems are erect, very long and narrow, looking 

 like thin limp ribbons, which are only kept in their erect position by the water. 

 The Zostera grows in large patches on the shore between tide-levels. Its leaves 

 are collected, and dried and under the name of Sea -grass are used as stuffing 

 for cushions. To this group belongs also Vallisneria spiralis, which is figured 

 opposite, and to the flowers of which we shall return in detail later on; lastly, 

 we may mention certain species of Sj^arganium. In addition to this gi'oup is 

 a second, as a representative of which may be named the curious Lattice -leaf 

 plant {Aponogeton fenestrale or Ouvirandra fenestralis) inhabiting the waters 

 of Madagascar. Its short stems are buried in the mud; the leaves have short 

 stalks, and are not erect, but distributed in rosettes over the muddy bottom. 

 The green colour of their chlorophyll is almost entirely obscured by a reddish- 

 brown pigment; the parenchyma, which usually fills the meshes of the net- work 

 of strands, is absent, and the strands forming the framework of the leaf-blade 

 are covered only with a thin layer of chloi-ophyll-bearing cells, so that the whole 

 structure reminds one of a leaf which has fallen from a tree in autumn and has 

 been macerated under water, of which, after the falling away of the easily de- 

 composed parenchyma, only the net-work of strands remains. The Water-lilies 

 may serve for a type of the third group. Their stems ai-e short, rooted in the 

 mud, and send out leaves whose broad blades, often circular in outline, are borne 

 on very long stalks. The disc-shaped leaf-blades lie with their under side on 

 the surface of the water, while their upper surface is exposed to the air. The 

 leaf-stalks thus traverse the whole depth of the water, and look like ropes by 

 which the floating leaf-discs are anchored in the muddy bottom. The long scapes, 

 terminating in floating flowers, serve a similar purpose. Here also must be 

 included the aquatic fem-like plant — Marsilea. Its leaves remind one of those of 

 the Wood Sorrel. The Frog-bit (Hydrocharis) and the Villarsia (Limnanthemum) 

 form a fourth group, not unlike water-lilies on a small scale. Their leaves and 

 flowers, however, do not arise directly from the main stem (as in the last group), 

 but from long lateral shoots, quite bai-e of leaves, till just close to the surface (c/. 

 vol. II., fig. 419). Our fifth group includes forms transitional between the groups 

 already described and the sixth and largest group. They include forms with 



